This invention concerns a system to re-circulate treatment material in processes of surface treatment and finishing.
The re-circulation system according to the invention is employed advantageously for the re-circulation of corrosive and abrasive acid baths or for the re-circulation of molten metals.
To be more exact, the system to re-circulate treatment material according to the invention is applied to pickling plants and/or turbulent pickling plants and/or plants for pickling and electrolytic processes employed for surface treatment of metallic objects such as metallic strip.
This system to re-circulate treatment material according to the invention is used advantageously also in plants for the surface finishing of metallic objects and especially in plants in which metallic coatings applied with heat, such as galvanising and aluminising, are produced.
The state of the art includes the pickling method which has the purpose of removing from the surface of the steel the non-metallic particles and, in particular, the oxides forming on that surface.
The pickling is carried out by making the surface of the steel cooperate with inorganic acids or salts which attack the surface of the steel to be treated in a more or less intense manner.
Pickling plants for strip, with which the invention is concerned, are continuous dip plants and comprise at least one dip tank filled with a solution containing the required acid or salt in which the strip, or material to be cleaned, is immersed and caused to pass at the desired speed.
The solution generally leaves the dip tank by gravity and is collected in an underlying re-circulation vessel, whence it is drawn by re-circulation pump means so as to be re-immitted into the dip tank.
The aggressive nature of the acid solutions used in the step of pickling or acid washing of the metal reduces considerably the life of the re-circulation pumps, which therefore have to be often replaced.
With a view to increasing the life of the pumps there is also a tendency to keep the temperature of the solution at values lower than those providing maximum efficiency of the pickling process.
Moreover, the presence of abrasive particles consisting mainly of scales of metallic oxides which are detached from the metallic surface makes the working conditions of the re-circulation pumps still more severe and shortens their life even further.
This situation makes necessary a frequent operation of filtration and/or regeneration of the acid solution used in the tanks together with related costs that increase the treatment costs.
Furthermore the surface finishing plants in which the metallic coating processes with heat, such as galvanising and aluminising for instance, are carried on entail great problems with regard to the movement and re-circulation of the molten metal, which in this case is zinc or aluminium; these problems are mainly due to the high temperature and great viscosity of the molten metal.
JP-A-1-36780 and DE-A-2.711,814 disclose systems to re-circulate a solution which employ re-circulation pumps and therefore entail all the above problems.
GB-A-2,055,771 discloses an autoclave system associated with a pickling plant and performing the emptying and filling of the pickling tank. This system includes the immersion of the objects to be pickled in a tank containing an unmoving acid solution and does not allow this solution to flow in countercurrent to the materials to be treated. Moreover, this system does not enable the acid solution to be re-circulated into the tank but merely makes possible the replacement of the solution consumed, thus ensuring a substantially constant level in the dip tank.
EP-A-0.402.270 discloses a method and plant to galvanise continuously or discontinuously objects which are passed into a sealed chamber containing the galvanising solution. This chamber includes upstream and downstream closure means which enable the chamber to be kept always full of galvanising solution at a preset pressure. The sealed chamber is associated with a tank under pressure, which ensures the presence of galvanising solution in the chamber; this system permits the replacement of the solution consumed but does not make possible the continuous re-circulation of the solution. Furthermore, the immersion chamber requires upstream and downstream closure means of an electromagnetic type, which are expensive and make the structure complicated.
DE-A-2.029.244 discloses a system for the filling and emptying of a tank containing a treatment solution at the beginning and end of processing but does not enable this solution to be re-circulated continuously.